In working with resistors referred to as “precision resistors”, it is advantageous to have the capability to precisely adjust the resistance value. It is also advantageous to precisely adjust the temperature coefficient of resistance (TCR) of such a resistor.
It is known that joint and independent adjustment of resistance and TCR can be achieved for compound resistors containing a first portion with a first resistance value and a positive TCR and a second portion with a second resistance value and a negative TCR (U.S. Pat. No. 4,079,349, U.S. Pat. No. 4,907,341, U.S. Pat. No. 6,097,276). Independent trimming of these two portions of the compound resistor results in the adjustment of the total resistance and of the TCR of the compound resistor. The trimming technique is based on a process of cutting the resistive material such as by laser beam cutting or ultrasonic probe cutting or others. The material properties, namely the TCR, of the bulk resistor material, remain substantially constant during this trimming process, since only the shapes of the resistor portions are being trimmed.
Another non-laser trimming technique is known to adjust the resistance of thin film resistors. This technique is based on thermal trimming of a resistor made from a thermally mutable material. Resistor trimming is achieved by heating using electric current pulses passed through the resistor itself or through an adjacent auxiliary heater (U.S. Pat. No. 4,210,996, 5,635,893, 5,679,275). Instead of direct physical removal of portions of the resistor material as is done in laser trimming, thermal trimming directly modifies the physical properties of the material such as resistivity and TCR.
It was reported that resistance trimming is accompanied by significant changes of TCR (K. Kato, T. Ono Changes in Thermal Coefficient of Resistance of Heavily Doped Polysilicon Resistors Caused by Electrical Trimming Jpn. J. Appl. Phys. Vol. 35 (1996), pp. 4209-4215; D. Feldbaumer, J. Babcock, C. Chen Pulse Current Trimming of Polysilicon Resistors Trans. On Electron Devices vol. 42 (1995), pp. 689-696; U.S. Pat. No. 6,306,718). As a measure of this effect, the term “Temperature Coefficient of Trimming” (TCT) is used hereinafter in this text and defines a change of TCR per fraction of trimming, which is a trimming-induced shift of TCR. For example, a TCT of −100 ppm/K/trim-fraction means that trimming resistance down by a trim fraction of 0.01 (1%) results in a shift of TCR equal to 10 ppm/K in the opposite direction from the direction of the trim (in this case an increase of 10 ppm/K). It is known experimentally that for polysilicon resistors, TCT is typically negative (increase in TCR with decrease of resistance), with its value dependent on type of dopant and doping level.
Non-zero TCT generates a new problem (not existing in typical cutting-based trimming techniques), which can be illustrated by the following example. Consider a resistor divider consisting of two trimmable resistors with the same initial TCR, and TCT=−2000 ppm/K/trim-fraction. If the resistance ratio is adjusted by trimming one of the resistors “down” by 10%, the accompanying change in relative TCR (RTCR) may reach 200 ppm/K. While resistance matching can potentially be done very precisely using thermal trimming (better than 0.01-0.1%), variation of ambient temperature in the range of ±50° C. can make the divider voltage very unstable, with resistance ratio drift reaching ±1%.
Near-zero TCR of the resistor is often desirable because it gives near-zero resistance drift with variation of ambient temperature. One of the problems of compound resistors consisting of two portions with positive and negative TCR is that near-zero TCR of the whole resistor does not mean near-zero TCR of each individual portion.
Thermally-trimmable resistors feature electrically-driven trimming, may be done at any practical stage in the manufacturer-to-user chain (including after packaging), and once trimmed, they are purely-passive components. However, they may also have constraints on bi-directionality of trimming. These types of resistors may often be more easily trimmable in one direction than another. For example, certain thermally-trimmable polysilicon resistors may be readily trimmed down (in the direction of decreasing resistance), from its as-manufactured resistance value (Ras-mfr), by tens of percent of Ras-mfr, but after such a trim-down, may have only limited trim-up (recovery) range. Other trimming characteristics also exist.
Joint and independent adjustment of resistance and TCR can be achieved for compound resistors containing a first portion with a first resistance value and a positive TCR and a second portion with a second resistance value and a negative TCR. Independent trimming of these two portions of the compound resistor results in the adjustment of the total resistance and of the TCR of the compound resistor.
In an in-circuit trimming application, limitations to full bi-directionality significantly constrain what can be done, and/or constrain trimming performance (e.g. speed, precision, range). For example, if a single trimmable resistor (any trimming method), is trimmable only in one direction, then one must be more conservative in seeking the Vout target. One must approach more slowly, to make sure that circuit settling times are accounted for, and one must be mindful of the quantization of the resistance trim, since the next trim may jump too far. Another example, if more than one trimmable resistor is used, then the optimal position of one resistor may depend on the position of the other, and vice-versa, but trims must be done sequentially, and so an individual trim of one resistor may reach an interim (non-optimal) target while irreversibly overshooting the optimal target (which may not be known until another resistor (or resistors) has reached its optimal position, or close to its optimal position).
The objects of the present invention include, without limitation, overcoming above described limitations.